Method of making an improved erythemal phosphor



June 20, 1950 H. c. FROELICH METHOD OF MAKING AN IMPROVED ERYTHEMAL PHOSPHOR Filed Aug. 2, 1945 M MRMERQMKkMXxQDQZM M H L m N M a. FULL WUR mg m T W E e N L F L E N 5 W w. Wm? M 0. My 1 PM? F/G. 3. 5o

475 AVEL aver/45 Patented June 20, 1950 "UNITED STATES PATENT OFFICE METHOD OF MAKING AN IMPROVED ERYTHEMAL PHOSPHOR Herman C. Froelich, Cleveland Heights, Ohio, assignor to General Electric Company, a corporation of New York Application August 2, 1945, Serial N 0. 608,487 8 Claims. (Cl. 252 301.4)

This invention relates to an erythemal gen- 40 per cent. These figures are for the radiation erator or sun lamp of fluorescent type, comof the phosphor alone, and not for that from a prising an electric discharge device p du g mercury vapor lamp containing the phosphor. radiation and a fluorescent composition or ph s As a means of converting electrical energy into phor exposed to its output and excited thereby erythemal radiation, a fluorescent lamp such as to radiate in the ultraviolet, and particularly in above mentioned containing my phosphor gives the erythemal range, which extends from about between five and six times the' efiiciency of the 800 A. to about 3200 A, Such an erythemal sun lamp known commercially as the S4. My lamp nd phosphor are described in appl a on product also shows the same essential freedom SerialNo. 488,885 of Willard A. Roberts, filed .10 from visible light emission that characterizes the May 28, 1943, and'assigned to the assignee of Roberts phosphor. By this I mean that there is this application, now Patent No. 2,4,47,210 dated no light from the phosphor itself (apart from the August 17, 194.8. The phosphor there described exciting shortwave source) that is perceptible to comprises a matrix of alkaline earth metal phosthe eye, and that at most the luminous output pha e p ally calcium orthophosphate) actiis of the rder of only about one per cent of the vated with thallium, and the discharge device total radiant output, with which it is associated is of the low pressure, To secure the high efliciency above referred to positive column, mercury vapor type employed as characteristic of my product in favorable in ordinary commercial fluorescent lamps, and specimens, it is not enough merely to introduce broadly exemplified in U. S. Patents Nos. 2,182,732 or mix a thallium ingredient (whether metal or to Meyer, Spanner, and Germer; 2,259,040 to compound) into a batch of previously prepared Inman; 2,306,925 to Aicher; and 2,312,245 to alkaline earth metal phosphate of suitable purity, Flaws. Such discharge devices are eflicient genand'to heat to a temperature such as hereinafter craters of shortwave ultraviolet radiation, and indicated. In my process, the phosphor is inparticularly of the 2537 A. resonant radiation deed prepared by a method which involves heatof mercury, by which the phosphor in question ing and calcining phosphate of alkaline earth is excited ina nearly maximum degree. For metal as the matrix material in the effective erythemal purposes, the envelopes of such lamps presence ofthallium as the activator; but preferare made of glass which transmits ultraviolet of ably (as also in Roberts process), this alkaline more than 2800 wave length while absorbing ear h metal phosp a s n si ed in the efthe shorter wave lengths that are harmful to the fective presence of the thallium, by reaction beeyes. The manufacture of the phosphor is also tween suitable ingredients comprising alkaline described in the said Roberts application. ea t metal a d the phosp oric radical -PO4, I have now discovered a new way of making before applying the degree of heat necessary to such phosphor or fluorescent material, whereby bri h all um o activating io o the its eificiency as a generator of erythemal ultrap osph e. mo g p p t s 0f e alkaline violet can be considerably increased. While the earth metals, my invention is especially conradiation of my product covers the same range cerned with the normal orthophosphate of calof ultraviolet as that of the Roberts product, its cium, Cas(PO4)2, as the matrix material. emission in the narrower erythemal portion of i0 Reaction of calcium and phosphoric compothis range can be made much stronger or brighter nents to yield only calcium orthophosphate and in the best-prepared specimens, partly by envolatiles is in itself well known in chemistry, as hancement of radiant intensity over the Whole well asthereagents that react in this Way. What ultraviolet range of the phosphon'and-partly behas not been thus known is that thedestiny of cause the peak intensity of the emission falls i; the product as a thallium-activated phosphor closer to the erythemal range. In comparison calls for special conditions in the synthesis: that with a quantum efiiciency of some per cent for it should take place in the Very presence of the the total radiation from favorable specimens of activating thallium; and that the calcium and the Roberts product, and of some 15 to 20 per phosphoric components should yield up their cent for the radiation in the erythemal range of 50 volatiles and interact at relatively low tempera- 2800 to 3200 A., favorable specimens of my prod tures. Here, a ain, the break-down temperauct show a total quantum efficiency of no less tures of various calcium and phosphoric comthan higher than for any phosphor now pounds are well known; but it has not been known used in fluorescent lamps, and an efiiciency for that low temperatures of break-down and interthe erythemal range that is of the order of30 to action are essential to give the more eiificient thallium activated calcium orthophosphate. The interaction of the calcium and phosphoric components takes place at higher or lower temperatures according as their individual breakdown temperatures or resistance to heat are higher or lower. Any calcium and phosphoric compounds that break down and interact at temperatures of the order of 200 C. or lower are suitable, according to my experience, when they yield only volatiles in addition to the calcium phosphate.

As described in the Roberts application, the matrix-forming components are calcium nitrate and diammonium phosphate, Ca(NO3)2-4I-I2O and (NHUzHPOi, whose reaction to produce the calcium phosphate Cas(PO4)2 liberates N02, H20, and NH3 to escape as gas or vapor. Besides using an excess of calcium nitrate which yields practicable with reactants such as I have indicated, more thallium can be retained in the phosphor; or, if desired, a higher degree of heat can be used in activating or forming the phosphor without incurring excessive loss of thallium. Indeed, I have found that the absence of any excess of the calcium component allows of having in the phosphor a greater proportion of retained thallium than is necessary to give maximum initial brightness; and I have found that this excess of retained thallium reduces the loss of brightness which the phosphor sustains in fluorescent lamp manufacture, and especially in the usual grinding ofthe phosphor with the organic binder used in applying it to lamp envelopes. On the other hand, an excess of the phosphoric component acts as a flux and tends to agglomeration and sinterlng, and also tends excess CaO in the product, Roberts alsouses sulphuric acid (H2SO4) in excess of that which reacts with theexcess CaO, this excess of acid also passing oil as gas or vapor. And he points out that without the use of sulphuric acid the phosphor ultimately produced shows only about the brightness of his product.

I have found that this necessity of using sulphuric acid in the Roberts process arises from the character of .the reactants, and especially from the use of such a substance as calcium nitrate. In the earlier stages of heating, at any rate, Ca(NO3)z exerts a deleterious influence, partly by acting as a flux or agglomerating agent as compared with products obtainable according to my invention. This is because the calcium nitrate does not break down or react with the phosphoric component at ordinary room temperatures of some C., or even in the earlier stages of heating, but only at a much higher temperature. The correspondingly high intrinsic resistance of calcium nitrate to thermal decomposition is shown by the relatively high melting point of 561 C. for the anhydrous substance.

As already intimated, I have found that a phosphor product showing the marked superiority above indicated can be obtained by employing calcium and phosphoric components which decompose and interact at relatively low temperatures, much lower than calcium nitrate. At the same time, the need for an excess of CaO or of any matrixiorming reactant is obviated, as well as that of using sulphuric acid or the like, and a phosphor of much finer grain size is obtained, as fine as a 2 to 4 micron particle size or diameter, which requires no reductive grinding to prepare it for use. A further advantage is greater uniformity or reproducibility of desired qualities in different batches of the product. The employment of a more readily decomposable and reactant phosphoric component than diammonium phosphate also contributes to all this, as well as the avoidance of heat-resistant calcium components, and the stoichiometric character of the matrix.

Thallium and its compounds are rather volatile, so that much of the thallium component used with the calcium and phosphoric components is lost in the heating to synthesize and activate the matrix. I have found, however, that by producing a matrix of stoichiometric normal calcium orthophosphate without an excess of either the phosphoric or the calcium component, as becomes to the formation of pyrophosphate instead of orthophosphate, with a product of inferior brightness.

The essential identity and emission of the phosphor depend on its being the orthophosphate rather than any other phosphate, whether more acid than ortho, like pyrophosphate or metaphosphate, or more basic, like apatiteor tetraphosphate. Its eiiiciency depends on its not being overheated in forming it. A metaphosphate or a pyrophosphate phosphor activated with thallium not only gives little or no ultraviolet, but is characterized by the visible light output of violet blue color described in U. S. Patent No. 2,270,124 to Huniger 8; Panke, which is so bright as to be termed medium in comparison with other blues that are termed very good or good. On the other hand, a thalliumactivated orthophosphate that is heated high enough to fuse or sinter it has virtually no emission of any kind, ultraviolet or visible. For optimum brightness of my product over its characteristic range of ultraviolet emission, which includes the erythem'al range above mentioned, the temperature of firing should be of the order of -10 00 C., or about 950 to 1100 C., with a preference for approximately or substantially 1000 to 1030 C. This is high enough to sinte or even fuse a metaphosphate or pyrophosphate phosphor as described in the Huniger and Panke patent, but isby no means-suiiicient toeven sinter my orthophosphate product, much less to fuse it.

The ingredients used in compounding the phosphor should preferably be free of any undesired components that will not be expelled by volatilization when they are mixed together, or

in the early stages of heating, in order to avoid any need for special purification during or after heating.

phosphor and prevent fluorescence is very important. This is especiallyso in the case of the thalliu'm"- component, because the generally un avoidable loss of thallium or its compounds by volatilization during heating greatly increases the proportionand effect of small amounts or traces of deleterious metals on theretained thallium; a 50 per cent improvement in total quantum efficiency has been obtained by purifying the thallium component of lead, silver, copper, or

other metals that are poisons against ultraviolet fluorescence. To obtain high brightness, the ingredients should be brought together inthe wet way, partly at least in solution or dispersion, While the particles of the solid ingredient(s)- should be as fine as possible.

As regards the calcium component, calcium oxide itself is unsuitable for mixing with a phos-- phoric component in the wet way, because of the uncontrollable violence of the resulting reaction; but I have found that the reaction is much gen tler with calcium compounds wherein the oxide 09.0 is bound up with components that readily volatilize out of the mixture at room temperature, or in the early stages of heating, these compounds being such as react with the phosphoric component at correspondingly low temperatures, such as some 260 C., or thereabout, as compared with more than 561 C. for calcium nitrate. Especially suitable are calcium hydroxide, Ca(QH)2, and calcium carbonate, CaCOa, which give. the brightest phosphors. As contrasted with the stability of calcium nitrate in the mixture, as above mentioned, calcium hydroxide, carbonate, and formate all with the phosphoric component react below 200 C. or even about 150 0., at room temperature or in the early stages of heating. However, phosphors made with calcium formats are not so bright, and those made with calcium acetate and oxalate are still poorer, as are many or all other compounds of this nature, which break down at temperatures well above 200 C.

For the phosphoric component, likewise, those compounds are to be preferred wherein the phosphate radical --PO4 is bound up only with components that readily volatilize out of the mixture at room temperatures, or in the early stages of heating, below about 200 C. or even 150 0., these compounds being themselves decomposable at correspondingly low temperatures such as some 2S0 0., or thereabout, as compared with more than 561 C. for calcium nitrate. Amongst such compounds, orthophosphoric acid or hydrogen phosphate, H3PO4, has given the brightest phosphors; but products nearly if not quite as good can be prepared with mono or diethyl phosphate, (C2H5)H2PO4 or (C2I-I5)2HPO4, or with the commercial mixture of them that is known in the trade as ethyl phosphoric acid; or with monoammonium phosphate, (NH4)H2PO4. Diammonium phosphate I (NHUZHPO'i gives a coarser product of inferior brightness. In general, any orthophosphoric esters give good phosphors provided they are water-miscible or soluble and yield by hydrolysis, when the ingredients are brought together in the wet way, phosphoric acid and an alcohol that is sufiiciently low-boiling and volatile to go off without charring when the mixture of calcium and phosphoric components is heated to a temperature not exceeding substantially 200 0., such phosphoric esters being in themselves well known to chemists. On the other hand, substances like sodium phosphate would yield an unwanted non-volatile reaction product diflicult to wash out completely.

The selection of a thallium. component or ingradient offers no problems, since it is the intimate combination of thallium atoms as such with the phosphate structure, by heating, that confers fluorescence, and not any chemical combination in which the thallium is employed. In general, any thallium compound may be used, though of course such as would introduce unwanted separate substances into the product will be avoided in practice, if only to prevent dilution of the phosphor. Accordingly, the bases ofpreference or choice are merely such as are naturally obvious to skilled chemists. Water-soluble compounds are preferable to free metal. or other insoluble forms of thallium, because it is easier to incorporate substances in liquid dispersion (i. e., in solution or in suspension) intimately and uniformly throughout the matrix-forming material.

However, this drawback of insolubles is somewhat mitigated by the volatilization of the thallium component during the heating. If it should be desired to use thallium metal as an ingredient and to avoid having it as oxide in the product, the heating of the mixture might be done in an inert environment, such as afforded by an evacuated and hermetically sealed quartz container, although such complications are not generally to be preferred.

The proportions of calcium and phosphoric components used should correspond stoichiometrically to the desired orthophosphate, without excess of either CaO or P205 in the mixture.

Provided the proportion of thallium in the final product is sufiicient, the proportion in the original mixture is not critical. A minor percentage is generally enough to assure an adequate amount in the product. The minimum percentage of retained thallium for maximum brightness of the phosphor itself is substantially 0.6 per cent by weight; but to assure the best practicable brightness in a fluorescent lamp, a greater proportion is desirable: for example, a phosphor with 2 to 4 per cent of retained thallium suffers only about one tenth the loss of brightness in grinding with binder that a phosphor having but 0.6% of thallium undergoes. 1

As a concrete illustration of the process, for the convenience of those desiring to use my invention, I will now give a specific example in detail; but this is not to be understood as defining or limiting the invention in its broader aspects.

As a preliminary step, 3 mole of calcium oxide (CaO) amounting to 168 grams, may be quenched or slaked with enough distilled water to give astifi paste. The calcium oxide should be of the finest possible grain size, since the grain size of the ultimate phosphor and its flourescent brightness partly depend on this. I have found that the slaking reaction can be controlled and made more gentle by moistening and even saturating the calcium oxide with acetone before adding the water, and that this tends to prevent coarsening of the grain size in the slaking. The acetone is largely volatilized and driven off by the heat of the reaction, or is at any rate expelled in the heating which the product afterward undergoes. After the reaction is complete, the viscous paste is thinned to a homogeneous thin paste by mixing in more water. It is undesirable to, add all the water to the calcium oxide initially, because this tends to coarsen the slaked product and to impair the brightness of the finished phorphor.

An aqueous solution of a soluble thallium compound such, as thallous hydroxide, TlOH, or thallium phosphate, preferably TlH2PO4, is added to the calcium hydroxide paste and thoroughly and homogeneously mixed in. The

amount of thallium thus added may be 10 to 30 grams or more, and a strength of the solution corresponding to a volume of 70 to 200 co. (more or less) is suitable. Then a 30 to percent aqueous solution of orthophosphoric acid H3P04.

is gradually added under constant stirring, the

amount of phosphoric acid being just sufficientto assure that all metal is brought into the form.

of orthophosphate. When thallium phosphate is used in the batch, the added phosphoric acid solution may consist of 231 grams of 85 per cent commercial reagent grade phosphoric acid further diluted with water to 400 to 600 00.; while when thallium hydroxide is employed, the amount of phosphoric acid may be correspondingly more than 231 grams. Mon-ammonium phosphate, NH4H2P04, may be substituted for H3PO4 with some sacrifice of brightness in the resulting phosphor; diammonium phosphate, (NHilZHPOt, gives a product of considerably lower brightness and coarser grain size.

The neutralizing reaction between the lime paste and the phorphoric acid develops considerable heat; but as this is not enough to dry the product, external heat is applied to dry it out till it can be crushed and sieved or ball-milled for a short time, so as to break up any agglomerations. It is not preferred to filter oif any of the superfiuous water, because this would remove any substances in solution, and might destroy the stoichiometric relation between calcium and phosphoric components besides wasting thallium. The dry powder is charged and packed into a covered crucible (as of porcelain or alundum) and fired,'as in an electric muille furnace, at about 1000 to 1100 C., preferably 1000 to 1030 C. The time of firing depends on the temperature and the amount of the charge: for 1000 to 1030 C. and 25 grams, a period of 40 min. is about right, while a larger charge or a lower temperature requires longer, and vice-versa. As a considerable amount of thallium component volatilizes and is lost during the firing, a few trials may be necessary to determine the very best proportions of this component and the optimum conditions and duration in each particular case.

After cooling, the phosphor is readyfor use. It can be coated on the inside of a lamp envelope with the aid of a carbonaceous binder in the usual way, though the grinding to incorporate the powder with the binder should be brief. To give the highest erythemal output, the phosphor coating on the lamp tube should be thinner than is usual for ordinary fiour escent tubes.

In the drawings, Fig 1 is a diagram of curves representing the special distribution of the radiation from 15-watt flourescent lamps having their envelopes internally coated with thallium activated calcium phosphate phosphors, the solid curve being for phosphor prepared as herein described, and the dotted or dash curve for phosphor prepared as described in the aforementioned Roberts application.

Fig. 2 showscurves of erythemal effect, obtained by multiplying the values of radiant energy for various wave-lengths in Fig. 1 by the known sensitiveness of the skin to energy of these wavelengths, and plotting the products as ordinates.

Fig. 3 shows a curve of excitability of the phosphor prepared as herein described by ultraviolet radiation of various wavelengths.

As "shown 'in Fig. '1, the radiation from both the old and the new phosphors starts from substantially zero at a wavelength somewhat below 2800 A, the lower limit of the erythexnal range, peaks around 3260 A. for the new phosphor as against about 3300 A. for the old, and falls to a relatively low value at 3800 A, and practically to zero at4000 A, which is generallyconsidered the lower limit of the visible. 3200 A. being the upper limitofthe 'eryth'emal, the seemingly small shift of the peak of radiant energy toward this value for the new phosphor results in an increase in its real erythemal effect considerably more than proportionate to the greater height of the peak, and seemingly very disproportionate to the magnitude-ofthe shift. This is strikingly shown by the greater ratio of peakvalues of the solid and dash curves in Fig. 2 than in Fig. 1, and still more We. comparison of the areas under the solid and dash curves in Figs. 2 and 1.

Fig. 3 illustrates another'important property of this phosphor, which is largely responsible for its very high efficiency in low pressure mercury vapor discharge devices: viz., the fact that the peak of its excitability at about 2475 A. falls so close to the resonance radiation of mercury at 2537 A, to which the phosphor is thus particularly-responsive.

What I- claim as new and desire to secure by Letters Patent of the United States is:

1. The method of preparing fluorescent thalhum-activated calcium orthophosphate which consists of preliminarily reacting at a temperature not exceeding approximately 200 C. a mixtureof calcium compound and phosphoric component in stoichiometri c proportions of calcium orthophosphate together with sufficient activating thallium compound to give between about 0.6% and 4% thallium in the final product, the calcium compound being of the group consisting of the hydroxide and carbonate and the phosphoric component'being of the group consisting of phosphoric acid, ethyl phosphate and ammonium phosphate, drying and comminuting the resultant product and then firing it at a temperature of approximately 950-1100 C. to complete the reaction and activatingly combine the thallium with the orthophosphate structure.

2. The method of preparing fluorescent thallium-activated calcium orthophosphate which consists of-preliminarily reacting at a temperature not exceeding approximately 200 C. a mixture of calcium hydroxide and orthophosphoric acid in stoichiometric proportions of calcium orthophosphate together with suiilcient activating thallium compound to give between about 0.6% and 4% thallium in the final product, drying and-comminuting the resultant product and then firing it at a temperature of approximately 950-1100 C. to "complete the reaction and activatingly combine the thallium with the orthophosphate structure.

- 3. The methodo'i preparing fluorescent thallium-activated calcium orthophosphate which consists of preliminarily reacting at a temperature not exceeding approximately 200 C. a mixture of calcium hydroxide and ethyl phosphate in stoichiometric proportions of calcium orthophosphate together -With sufiicient activating thallium compound to give-between about 0.6% and 4% thallium in'the final product, drying and comminuting the resultant product and then firing it at a temperature of approximately 950 1100 C. to complete the reaction and activatingly combine the thallium with the orthophosphate structure.

4. The method of preparing fluorescent thallium-activated calcium orthophosphate which consists of preliminarily reacting at a temperature not exceeding approximately 200 C. a mixtureof calcium hydroxide and ammonium phosphate in stoichiometric proportions of calcium orthophosphate together with sufiicient activating thallium compound to give between about 0.6% and 4% thallium in the final product, drythen firing it at a temperature of approximately 950-1100 C. to complete the reaction and activatingly combine the thallium with the orthophosphate structure. 7

6. The method of preparing fluorescent thallium-activated calcium orthophosphate which consists of preliminarily reacting at a temperature not exceeding approximately 200 C. a mixture of calcium carbonate and ethyl phosphate in stoichiometric proportions of calcium orthophosphate together with suflicient activating thallium compound to give between about 0.6% and 4% thallium in the final product, drying and comminuting the resultant product and then firing it at a temperature of approximately 950-1100 C. to complete the reaction and activatingly combine the thallium with the orthophosphate structure.

7. The method of preparing fluorescent thallium-activated calcium orthophosphate which consists of preliminarily reacting at a temperature not exceeding approximately 200 C. a mixture of calcium carbonate and ammonium phosphate in stoichiometric proportions of calcium orthophosphate together with sufiicient activating thallium compound to give between about 0.6% and 4% thallium in the final product, drying and comminuting the resultant product and then firing it at a temperature of approximately 950- 1100 C. to complete the reaction and activatingly combine the thallium with the orthophosphate structure.

8. The method of preparing fluorescent thallium-activated calcium orthophosphate which consists of quenching calcium oxide with sufficient water to give a stiff paste and, after the reaction is complete, adding more waterto give a thin paste, adding suificient thallium compound to give between about 0.6% and 4% thallium in the final product, adding an aqueous solution of orthophosphoric acid in an amount sufilcient to give calcium orthophos'phate, drying and comminuting the resultant product and then firing it at a temperature of approximately 9501100 C. to complete the reaction and activatingly combine the thallium with the orthophosphate structure.

HERMAN C. FROELICH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,257,667 Aschermann Sept. 30, 1941 2,270,124 Huniger Jan. 13, 1942 2,306,567 Roberts Dec. 29, 1942 2,306,626 Huniger Dec. 29, 1942 2,447,210 Roberts Aug. 17, 1948 OTHER REFERENCES Langes Handbook of Chemistry, fourth edition, 1941.

Certificate of Correction Patent No. 2,512,270 June 20, 1950 HERMAN G. FROELICH It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 5, line 24, strike out the words with the phosphoric component and insert the same after react same line; column 7, line 49, for special read spectral;

and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in the Patent Oflice. Signed and sealed this 27th day of February, A. D. 1951.

THOMAS F. MURPHY,

Assistant (J'ommiaaioner of Patents. 

1. THE METHOD OF PREPARING FLUORESCENT THALLIUM-ACTIVATED CALCIUM ORTHOPHOSPHATE WHICH CONSISTS OF PRELIMINARY REACTING AT A TEMPERATURE NOT EXCEEDING APPROXIMATELY 200*C. A MIXTURE OF CALCIUM COMPOUND AND PHOSPHORIC COMPONENT IN STOICHIOMETRIC PROPORTIONS OF CALCIUM ORTHOPHOSPHATE TOGETHER WITH SUFFICIENT ACTIVATING THALLIUM COMPOUND TO GIVE BETWEEN ABOUT 0.6% AND 4% THALLIUM IN THE FINAL PRODUCT, THE CALCIUM COMPOUND BEING OF THE GROUP CONSISTING OF THE HYDROXIDE AND CARBONATE AND THE PHOSPHORIC COMPONENT BEING OF THE GROUP CONSISTING OF PHOSPHORIC ACID, ETHYL PHOSPHATE AND AMMONIUM PHOSPHATE, DRYING AND COMMINUTING THE RESULTANT PRODUCT AND THEN FIRING IT AT A TEMPERATURE OF APPROXIMATELY 950-1100*C. TO COMPLETE THE REACTION AND ACTIVATINGLY COMBINE THE THALLIUM WITH THE ORTHOPHOSPHATE STRUCTURE. 